Could this be Corydalis claviculata (climbing corydalis)? It was found on 25 March is large swathes on a recently raised soil mound in a pine plantation. It seems to be too early for it to be flowering?

Yes it's Climbing Corydalis -Ceratocapnos (syn.Corydalis) claviculata. But remarkably early. Not normally in flower until May!
Even though it's rather local here, it's probably commoner in Britain than anywhere else.
An unmistakable species and a good find.
Dorts.

Though normally said to be an annual, it is said by some to exhibit a perrennial habit in some places which would certainly give it an advantage.
Though maybe a bit too delicate to be a 'woodland thug'.
Dorts.

Thanks to you all for this confirmation. I think that the 'woodland thug' is probably the holiday camp company which has commandeered the wood and has raised defences around it. It was this disturbed soil which has provided the right habitat for the plant to flourish, perhaps the seeds have lain dormant since the plantation was established, some 30-40 years ago?